The present invention relates to reflective screens. In particular, it relates to a wavelength-selective reflective screen onto which an image from a projector, such as a video projector, a film projector, or an overhead projector, is projected with excellent color reproducibility and a high contrast even in a bright environment.
Recent years have seen widespread use of overhead and slide projectors by presenters delivering their materials in conferences and the like. Liquid crystal video projectors and motion picture film projectors for home use are also gaining popularity. These projectors operate by optically modulating light emitted from a light source using, for example, a liquid crystal panel, to form imaging light and then throwing the imaging light through an optical system, such as a lens, onto a screen.
For example, a front projector that forms a color image on a screen includes an illuminating optical system for splitting light emitted from a light source into red (R), green (G), and blue (B) beams and converging these beams onto predetermined optical paths, a liquid crystal panel (light valves) for optically modulating the respective red, green, and blue beams split in the illuminating optical system, and an optical combinatory unit for combining the R, G, and B beams optically modulated with the liquid crystal panel. A color image produced in the optical combinatory unit is enlarged and projected onto a screen via a projection lens.
A more recent development provides a projector that uses a narrowband light source of three primary colors and a grating light valve (GLV) instead of the liquid crystal panel to spatially modulate the respective RGB beams.
The projectors described above use projection screens to provide images. Projection screens can be roughly categorized into transmissive screens and reflective screens. With transmissive screens, light is emitted from behind the screen to present images to users sitting in front of the screen. With reflective screens, light is emitted from the front of the screen to deliver users images formed by reflected light. In order for the screens of any type to exhibit high visibility, formation of bright, high-contrast images is desirable.
However, unlike emission displays or rear projectors, front projectors can rarely reduce reflection of ambient light using, for example, an ND filter. Thus, it has been difficult to increase the bright area contrast of the front projectors on reflective screens.
A proposal for overcoming this problem provides a reflective screen incorporating an optical thin film (optical laminate) including a dielectric laminate that exhibits high reflection properties to light in particular wavelength regions and high transmission properties at least to light in the visible wavelength region outside those particular wavelength regions. In this screen, the thicknesses of optical films constituting the dielectric laminate are designed by a matrix simulation. An example of such proposal is found in Japanese Unexamined Patent Application Publication No. 2003-270725.
In the above-described screen, the optical laminate serves as a spectrum filter, which reflects most of light in particular wavelength regions. For example, the optical laminate transmits most of incident ambient light outside those particular wavelength regions.
As is described above, this reflective screen can selectively reflect light of particular wavelengths and reduces the reflection of ambient light compared to typical screens. Accordingly, a decrease in contrast of images formed on the screen can be suppressed, reflection of ambient light can be effectively reduced, and bright images can be obtained. Moreover, according to this reflective screen, clear images can be obtained in a bright environment, and thus clear images can be produced irrespective of the brightness of the environment. In particular, remarkably high contrast can be achieved with light sources, such as GLV, having a steep spectrum with a half width smaller than the half width of the reflectance of the screen in particular wavelength regions. Thus, the capacity of the light source can be fully yielded.
An LCD projector incorporating a high-pressure mercury lamp (UHP lamp) suffers from poor white balance of the imaging light despite the use of the reflective screen described above.
A reflective screen that can adjust the balance among three primary colors (RGB) in the imaging light and increase the purity of RGB to thereby extend the color gamut is desired.